Spatial Vision: Primary Visual Cortex

(Chapter 3, part 1)

Lecture 6

Jonathan PillowSensation & Perception (PSY 345 / NEU 325)

Princeton University, Spring 2015

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Chapter 2 remnants

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Receptive field: “what makes a neuron fire”

• weighting function that the neuron uses to add up its inputs”

patch of light

1×(+5) + 1×(-4) = +1 spikes

light level

“center”weight

“surround”weight

+-+

++

+ -

-

-light=+1

Response to a dim light

ON cell3

+-+

++

+ -

-

-patch of bright light

1×(+5) + 0×(-4) = +5 spikes

light level

“center”weight

“surround”weight

Response to a spot of light

Receptive field: “what makes a neuron fire”

• weighting function that the neuron uses to add up its inputs”

ON cell4

Mach Bands! Each stripe has

constant luminance (“light level”)

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+-+

++

+ -

-

-light=+2

2×(+5) + 2×(-4) = +2 spikes

higher light level

“center”weight

“surround”weight

Response to a bright light

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+-+

++

+ -

-

-+2

Response to an edge

+1

“surround”weight

“center”weight

2×(+5) + 2×(-3) + 1×(-1) = +3 spikes

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+-+

++

+ -

-

-+2 +1

+2 +2 +2 +3 0 +1 +1 +1+2 +2 +2 +3 0 +1 +1 +1 +2 +2 +2 +3 0 +1 +1 +1 +2 +2 +2 +3 0 +1 +1 +1 +2 +2 +2 +3 0 +1 +1 +1 +2 +2 +2 +3 0 +1 +1 +1

Mach Band response

“surround”weight

“center”weight

2×(+5) + 2×(-3) + 1×(-1) = +3 spikes

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+-+

++

+ -

-

-+2 +1

+2 +2 +2 +3 0 +1 +1 +1+2 +2 +2 +3 0 +1 +1 +1 +2 +2 +2 +3 0 +1 +1 +1 +2 +2 +2 +3 0 +1 +1 +1 +2 +2 +2 +3 0 +1 +1 +1 +2 +2 +2 +3 0 +1 +1 +1

Mach Band response

“surround”weight

“center”weight

2×(+5) + 2×(-3) + 1×(-1) = +3 spikes

Response to an edge

edges are where light difference is greatest

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Lightness illusion

Also explains:

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Figure 2.12 Different types of retinal ganglion cells

Magnocellular(“big”, feed pathway processing

motion)

Parvocellular(“small”, feed pathway processing

shape, color)

ON and OFF retinal ganglion cells’ dendrites arborize (“extend”) in different layers:

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ON, P-cells (light, fine shape / color)

OFF, M-cells (dark stuff, big, moving)Incoming Light

ON, M-cells (light stuff, big, moving)

OFF, P-cells (dark, fine shape / color)

“Channels” in visual processing

thebrain

The Retina Optic Nerve

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the more light, the more photopigment gets “used up”,→ less available photopigment,→ retina becomes less sensitive

Two mechanisms for luminance adaptation (adaptation to levels of dark and light):

(1) Pupil dilation(2) Photoreceptors and their photopigment levels

remarkable things about the human visual system: • incredible range of luminance levels to which we can adapt

(six orders of magnitude, or 1million times difference)

Luminance adaptation

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The possible range of pupil sizes in bright illumination versus dark

• 16 times more light entering the eye

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Contrast = difference in light level, divided by overall light level

(Think back to Weber’s law!)

• It turns out: we’re pretty bad at estimating the overall light level.

• All we really need (from an evolutionary standpoint), is to be able to recognize objects regardless of the light level

• This can be done using light differences, also known as “contrast”.

Luminance adaptation- adaptation to light and dark

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+5-4 Contast is (roughly) what retinal neurons compute, taking the difference between light in the center and surround!

Luminance adaptation

• from an “image compression” standpoint, it’s better to just send information about local differences in light

“center-surround” receptive field

Contrast = difference in light level, divided by overall light level

(Think back to Weber’s law!)

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• transduction: changing energy from one state to another

• Retina: photoreceptors, opsins, chromophores, dark current, bipolar cells, retinal ganglion cells.

• “backward” design of the retina

• rods, cones; their relative concentrations in the eye

• Blind spot & “filling in”

• Receptive field

• ON / OFF, M / P channels in retina

• contrast, Mach band illusion

• Light adaptation: pupil dilation and photopigment cycling

summary: Chap 2

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Now that you know how the early visual system works....

a little update on futuristic technology:

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• 60 electrodes• future versions to have 200, 1000 electrodes

• shows patterns of light and dark, like the “pixelized image we see on a stadium scoreboard,”

http://www.nytimes.com/2013/02/15/health/fda-approves-technology-to-give-limited-vision-to-blind-people.html

Device Offers Partial Vision for the Blind (Feb 2013)

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http://www.nytimes.com/video/science/100000002039719/the-fda-approves-a-bionic-eye.html

[movie]

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Spatial Vision:From Stars to Stripes

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MotivationWe’ve now learned:• how the eye (like a camera) forms an image.• how the retina processes that image to extract contrast (with “center-surround” receptive fields)

Next:• how does the brain begin processing that information to extract a visual interpretation?

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early visual pathway

optic nerve

optic chiasm

optic tract

lateral geniculate nucleus (LGN)

optic radiations

primary visual cortex (“V1”)

thalamus:

cortex:

(aka “striate cortex”)right visual

worldleft visual

world

eye eye

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• Acuity: measure of finest visual detail that can be resolved

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Visual Acuity

• Acuity: The smallest spatial detail that can be resolved

• in the lab

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Measuring Visual Acuity

Snellen E test• Herman Snellen invented this method for designating visual acuity

in 1862

• Notice that the strokes on the E form a small grating pattern

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Acuityeye doctor: 20 / 20 (your distance / avg person’s distance) for letter identification

vision scientist: visual angle of one cycle of the finest grating you can see

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• striped pattern is a “sine wave grating”

• visual system “samples” the grating at cone locations

explaining acuity stimulus on retina

percept

acuity limit: 1’ of arc

cone spacing in fovea: 0.5’ of arc

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more “channels”: spatial frequency channels

spatial frequency: the number of cycles of a grating per unit of visual angle (usually specified in degrees)• think of it as: # of bars per unit length

low frequency intermediate high frequency

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Visual Acuity:

Why sine gratings?• The visual system breaks down images into a vast

number of components; each is a sine wave grating with a particular spatial frequency

Technical term: Fourier decomposition

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• mathematical decomposition of an image (or sound) into sine waves.

Fourier decomposition

“image”1 sine wave

reconstruction:

2 sine waves

3 sine waves

4 sine waves

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“Fourier Decomposition” theory of V1

• Summation of two spatial sine waves• any pattern can be broken

down into a sum of sine waves

claim: role of V1 is to do “Fourier decomposition”, i.e., break images down into a sum of sine waves

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• mathematical decomposition of an image (or sound) into sine waves.

Fourier decomposition

Original image High Frequencies Low Frequencies

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original

low medium high

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Retinal Ganglion Cells: tuned to spatial frequency

Response of a ganglion cell to sine gratings of different frequencies

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The contrast sensitivity function

Human contrast sensitivity illustration of this sensitivity

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Image Illustrating Spatial Frequency Channels

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Image Illustrating Spatial Frequency Channels

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If it is hard to tell who this famous person is, try squinting or defocusing

“Lincoln illusion” Harmon & Jules 197340

“Gala Contemplating the Mediterranean Sea, which at 30 meters becomes the portrait of Abraham Lincoln (Homage to Rothko)”

- Salvador Dali (1976)

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- Salvador Dali (1976)

“Gala Contemplating the Mediterranean Sea, which at 30 meters becomes the portrait of Abraham Lincoln (Homage to Rothko)”

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Summary

• early visual pathway: retina -> LGN -> V1

• “contralateral” representations in visual pathway

• visual acuity (vs. sensitivity)

• spatial frequency channels

• Fourier analysis

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